plot Nuss over circle boundary

Question

Hi,
How to extract and plot over the subdomain boundary

from dolfin import*
from mshr import *

# Create mesh 
H =  0.41
Lstart = -0.2
Lend = 2.0
rc= Rectangle(Point(Lstart, 0.0), Point(Lend, 0.41))
c = Circle(Point(0.2,0.25), 0.1)
geometry = rc-c
mesh = generate_mesh(geometry, 10)

#def Function Spaces
V = VectorFunctionSpace(mesh, "CG", 2)
Q = FunctionSpace(mesh, "CG", 1)
W = V*Q

#def Boundary
class NoSlipDomain(SubDomain):
    def inside(self, x, on_boundary):
        return (near(x[1],0) or near(x[1],H) 
        or on_boundary and not inflow_boundary(x) and not outflow_boundary(x))

def inflow_boundary(x):
    return near(x[0],Lstart)

def outflow_boundary(x):
    return near(x[0],Lend)

noslip_boundary= NoSlipDomain()

#def Boundary condition
bc0 = DirichletBC(W.sub(0), (0.0,0.0), noslip_boundary)
inflow = Expression(("-sin(x[1]*pi)", "0.0"))
bc1 = DirichletBC(W.sub(0), inflow, inflow_boundary)
outflow = Constant(0.0)
bc2 = DirichletBC(W.sub(1), outflow, outflow_boundary )
bcs = [bc0, bc1, bc2]

#def variational forms
(u, p) = TrialFunctions(W)
(v, q) = TestFunctions(W)
f = Constant((0.0,0.0))
a = inner(grad(u), grad(v))*dx
L = inner(f,v)*dx
w = Function(W)

#solve problem
solve(a == L, w, bcs)
(u,p) = w.split(True)

#mesh function for circle boundary
boundary_parts = MeshFunction("size_t", mesh, mesh.topology().dim()-1)

Circle = AutoSubDomain(lambda x, on_boundary: on_boundary and not
                       (near(x[0], Lstart) or near(x[0], Lend) or near(x[1], 0.0) or near(x[1], H)))

Cirlce.mark(boundary_parts, 0)

n = FacetNormal(mesh)

#Projection of u to du/dn

du_n = TrialFunction(V)
phi = TestFunction(V)

#desired Function: du_n, integration only on surface of circle
a_n = inner(du_n, phi)*ds(0)

#gradient of velocity_field u  with normal vector field n
L_n = inner(dot(nabla_grad(u), n), phi)*ds(0)
Nuss = Function(V)
solve(a_n == L_n, Nuss)

Thank you
palraj